I've read 2 different versions, the first was that it was sign/message to the bosses on the street that the old way of doing things were over, with Costello supposedly being the largest opponent of drug dealing. The second version was that Costello/Gambino/Luciano had something to do with Genovese and Galante going to prison in the late 1950's/early 1960's, I'm not sure if their cases were intertwined but I guess it's plausible, Genovese was convicted in 1959 and Galante in 1961. I wish I had a source, I can't remember where I read it, possibly in the first edition of Carl Sifakis's Mafia Encyclopedia.